Method for treating thermoplastic polyurethane golf ball covers

ABSTRACT

A method of forming a golf ball is disclosed herein. The method includes placing a golf ball precursor product comprising a thermoplastic polyurethane cover further comprising an isocyante or hydroxyl terminated polybutadiene in an isocyanate solution. The precursor product is then removed and heated to remove solvent. The precursor product may then be subjected to gamma or electron beam irradiation.

CROSS REFERENCES TO RELATED APPLICATIONS

Not Applicable

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

Not Applicable

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a golf ball. More specifically, thepresent invention relates to a method for treating a thermoplasticpolyurethane golf ball cover.

2. Description of the Related Art

Traditional golf ball covers have been comprised of balata or blends ofbalata with elastomeric or plastic materials. The traditional balatacovers are relatively soft and flexible. Upon impact, the soft balatacovers compress against the surface of the club producing high spin.Consequently, the soft and flexible balata covers provide an experiencedgolfer with the ability to apply a spin to control the ball in flight inorder to produce a draw or a fade, or a backspin which causes the ballto “bite” or stop abruptly on contact with the green. Moreover, the softbalata covers produce a soft “feel” to the low handicap player. Suchplayability properties (workability, feel, etc.) are particularlyimportant in short iron play with low swing speeds and are exploitedsignificantly by relatively skilled players.

Despite all the benefits of balata, balata covered golf balls are easilycut and/or damaged if mis-hit. Golf balls produced with balata orbalata-containing cover compositions therefore have a relatively shortlife span.

As a result of this negative property, balata and its syntheticsubstitutes, trans-polybutadiene and transpolyisoprene, have beenessentially replaced as the cover materials of choice by other covermaterials such as ionomeric resins and polyurethanes.

Ionomeric resins are polymers containing interchain ionic bonding. As aresult of their toughness, durability and flight characteristics,various ionomeric resins sold by E.I. DuPont de Nemours & Company underthe trademark Surlyn® and by the Exxon Corporation (see U.S. Pat. No.4,911,451) under the trademarks Escor® and Iotek®, have become widelyutilized for the construction of golf ball covers over the traditional“balata” (transpolyisoprene, natural or synthetic) rubbers. As stated,the softer balata covers, although exhibiting enhanced playabilityproperties, lack the durability (cut and abrasion resistance, fatigueendurance, etc.) properties required for repetitive play.

Ionomeric resins are generally ionic copolymers of an olefin, such asethylene, and a metal salt of an unsaturated carboxylic acid, such asacrylic acid, methacrylic acid, or maleic acid. Metal ions, such assodium or zinc, are used to neutralize some portion of the acidic groupsin the copolymer resulting in a thermoplastic elastomer exhibitingenhanced properties, such as durability, for golf ball coverconstruction over balata. However, some of the advantages gained inincreased durability have been offset to some degree by the decreasesproduced in playability. This is because although the ionomeric resinsare very durable, they tend to be very hard when utilized for golf ballcover construction, and thus lack the degree of softness required toimpart the spin necessary to control the ball in flight. Since theionomeric resins are harder than balata, the ionomeric resin covers donot compress as much against the face of the club upon impact, therebyproducing less spin. In addition, the harder and more durable ionomericresins lack the “feel” characteristic associated with the softer balatarelated covers.

As a result, while there are many different commercial grades ofionomers available both from DuPont and Exxon, with a wide range ofproperties which vary according to the type and amount of metal cations,molecular weight, composition of the base resin (for example, relativecontent of ethylene and methacrylic and/or acrylic acid groups) andadditive ingredients such as reinforcement agents, etc., a great deal ofresearch continues in order to develop a golf ball cover compositionexhibiting not only the improved impact resistance and carrying distanceproperties produced by the “hard” ionomeric resins, but also theplayability (for example, “spin”, “feel”, etc.) characteristicspreviously associated with the “soft” balata covers, properties whichare still desired by the more skilled golfer.

Furthermore, a number of different golf ball constructions, such asone-piece, two-piece (a solid resilient center or core with a moldedcover), three-piece (a liquid or solid center, elastomeric winding aboutthe center, and a molded cover), and multi-piece golf balls, have beendeveloped to produce golf balls exhibiting enhanced playability anddurability. The different types of materials utilized to formulate thecores, mantles, windings, covers, etc. of these balls dramaticallyalters the balls' overall characteristics. In addition, multi-layeredcovers containing one or more ionomer resins or other materials havealso been formulated in an attempt to produce a golf ball having theoverall distance, playability and durability characteristics desired.

For example, in various attempts to produce a durable, high spin golfball, the golfing industry has blended the hard ionomer resins with anumber of softer ionomeric resins and applied these blends to two-pieceand three-piece golf balls. U.S. Pat. Nos. 4,884,814 and 5,120,791 aredirected to cover compositions containing blends of hard and softionomeric resins. However, it has been found that golf ball coversformed from hard-soft ionomer blends tend to become scuffed more readilythan covers made of hard ionomer alone. Consequently, it would be usefulto develop a golf ball having a combination of softness and durabilitywhich is better than the softness-durability combination of a golf ballcover made from a hard-soft ionomer blend.

Additionally, thermoset and thermoplastic polyurethanes have recentlybecome popular materials of choice for golf ball cover construction.However, these polyurethanes are difficult and time consuming toprocess. Moreover, the molding of relatively thin wall cover layer(s),i.e., cover layers 0.075 inches or less in cross-sectional thickness, isdifficult to accomplish. This limits the desired performance achieved bythin wall cover molding, such as improved distance. Furthermore, golfballs produced utilizing these materials tend to be soft and readilysusceptible to scuffing.

As a result, it would be further desirable to produce a thermoplasticpolyurethane covered golf ball having a thin wall cover constructionwhich exhibits enhanced durability, namely improved cut and scuff(groove shear) resistance, while maintaining and/or improving suchcharacteristics as playability and distance.

BRIEF SUMMARY OF THE INVENTION

The present invention provides a method of improving the durability,namely scuff resistance, of a golf ball with a thermoplastic cover. Theinvention is able to increase the durability of a golf ball with athermoplastic polyurethane cover through the addition of reactivemoieties to the cover prior to subsequent treatment with gammairradiation.

One aspect of the present invention is a method of forming a golf ballwith a thermoplastic polyurethane cover. The method includes placing agolf ball precursor product in a solution to create a solution coveredgolf ball precursor product. The solution includes a hydroxyl orisocyanate terminated polybutadiene in an amount of 10% to 30% by weightof the solution and a solvent. Next, the solution covered golf ballprecursor product is removed from the solution. Next, the solutioncovered golf ball precursor product may or may not be heated to removethe solvent. Next, the solution covered golf ball precursor product isplaced in an isocyanate solution to create a isocyanate solution coveredgolf ball precursor product. Next, the isocyanate solution covered golfball precursor product is heated to create a final golf ball precursorproduct. The final golf ball precursor product may then be send forgamma or electron beam irradiation.

Another aspect of the present invention is a method of forming a golfball. The method begins with molding a thermoplastic polyurethanematerial comprising a hydroxyl or isocyanate functional polybutadieneover a golf ball insert, forming a golf ball precursor product. Thefunctionalized polybutadience may be present in the cover material in anamount from 1% to 25% by weight. Furthermore, the polybutadiene materialmay be pre-compounded into the thermoplastic polyurethane or may beadded directly into the barrel of the molding apparatus or may beincorporated by any method know to those skilled in the art. The insertmay comprise any construction know to those skilled in the art,including, but not limited to solid cores, liquid cores, metal cores, ormulti-layer constructions. The golf ball precursor product preferablyincludes a core, a boundary layer and a cover comprising a thermoplasticpolyurethane material. Next, golf ball precursor product may be placedin an isocyanate solution to create an isocyanate solution covered golfball precursor product. The isocyanate solution includes acetone andMDI. Next, the isocyanate solution covered golf ball precursor productis heated to create a treated golf ball precursor product, wherein theisocyanate solution covered golf ball precursor product is heated fromtwo to four hours at a temperature ranging from 125° F. to 250° F. Thegolf ball precursor product or the treated precursor product is thenirradiated with gamma or electron beam radiation, forming a final golfball precursor product.

Still another aspect of the present invention is a golf ball having acover composed of a thermoplastic polyurethane and a functionalizedpolybutadiene material in an amount ranging from 1 to 3 weight percentof the cover. The cover can be used on a two-piece, three-piece ormultiple-piece golf ball. The thickness of the cover is preferably from0.020 inch to 0.060 inch, and more preferably from 0.030 inch to 0.045inch. The cover exhibits good scuff resistance.

Having briefly described the present invention, the above and furtherobjects, features and advantages thereof will be recognized by thoseskilled in the pertinent art from the following detailed description ofthe invention when taken in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

FIG. 1 is a flow chart of a preferred method of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

As shown in FIG. 1, a method of the present invention is generallydesignated 200. At block 202, a golf ball precursor product (referred toas “GBPP” in FIG. 1) is formed having a cover comprising a thermoplasticpolyurethane material. Alternatively, the cover comprises athermoplastic polyurethane material and a functionalized polybutadienematerial. Alternatively, the cover is composed of a blend of athermoplastic polyurethane material and a polyurea, ionomeric ornon-ionomeric material and a functionalized polybutadiene. In preferredembodiment, the golf ball precursor product is a three-piece solid golfball. Alternatively, the golf ball precursor product is a two-piece golfball with a thermoplastic polyurethane cover. Those skilled in thepertinent art will recognize other golf ball constructions thatincorporate a thermoplastic polyurethane cover.

If the cover is composed of a thermoplastic polyurethane without afunctionalized polybutadiene material, then at block 204, the golf ballprecursor product is placed in a solution. The solution includes ahydroxyl or isocyanate terminated polybutadiene in an amount of 10% to30% by weight of the solution and a solvent, and more preferably 20% byweight of the solution. A preferred material is a KRASOL LBH liquid lowmolecular weight polybutadiene with terminal secondary hydroxyl groups,which is available from Kaucuk of the Czech Republic. Another preferredmaterial is KRASOL NN22. The solution also preferably includes asolvent. A preferred solvent is acetone. Other solvents include methylethyl ketone and toluene. The golf ball precursor product is preferablyplaced in the solution for approximately one to two minutes.

At block 206, a solution covered golf ball precursor product is removedfrom the solution. At block 208, the solution covered golf ballprecursor product is heated to remove the solvent. Preferably, thesolution covered golf ball precursor product is heated at a temperatureranging from 125° F. to 250° F. for approximately two to four hours.Alternatively, the solution covered golf ball precursor product isallowed to air dry at room temperature (approximately 72° F.) for two tosix hours.

In an alternative embodiment, the solution dipping (blocks 204, 206 and208) of the golf ball precursor product is eliminated for a cover of thegolf ball precursor product which comprises a functionalizedpolybutadiene material incorporated during or prior to injection orcompression molding since the functionalized polybutadiene material isalready present in the cover.

At block 210, a golf ball precursor product comprising thefunctionalized polybutadiene is placed in an isocyanate solution forapproximately one to two minutes. The isocyanate solution can be anyaliphatic or aromatic isocyanate or diisocyanate or blends thereof knownin the art. The isocyanate or diisocyanate used preferably has a solidscontent in the range of about 1 to about 100 weight % of the isocyanatesolution, preferably about 5 to about 50 weight % of the isocyanatesolution, most preferably about 10 to about 30 weight % of theisocyanate solution. If it is necessary to adjust the solids content,any suitable solvent that will allow penetration of the isocyanate intothe polyurethane, polyurea or polyurethane/polyurea cover materialwithout causing distortion may be used. Examples of suitable solventsinclude ketone and acetate. Preferably, the isocyanate used is of theMDI type at 15 to 30% solids reduced with a ketone (such as Mondur ML™from Bayer Corporation) and dipped for 2 to 3 minutes. Most preferably,the solids level is about 16 to 24% (20±4). It is beneficial that theMDI remain in a liquid state at room temperature. However, this methodshall not be limited to the type of polyurethane, polyurea orpolyurethane/polyurea material, isocyanate used, concentration of theisocyanate solution, solvent used, dip time, or method of applicationdescribed above.

At block 212, the isocyanate solution covered golf ball precursorproduct is removed from the solution. At block 214, the isocyanatesolution covered golf ball precursor product is heated to remove thesolvent. Preferably, the isocyanate solution covered golf ball precursorproduct is heated at a temperature ranging from 125° F. to 250° F. forapproximately two to four hours. Alternatively, the isocyanate solutioncovered golf ball precursor product is allowed to air dry at roomtemperature (approximately 72° F.) for two to six hours. Optionally, atblock 216, the final golf ball precursor product is treated with gammaor electron beam irradiation. The final golf ball precursor product ispreferably treated with gamma irradiation at a level from 10 to 100kilograys. At block 218, the irradiated final golf ball precursorproduct or the final golf ball precursor product is finished bypreferably applying at least one coating layer and an indicia.

In a preferred embodiment, the cover is a multi-layer cover comprisingan inner cover layer or layers formed over the core. Preferably, theinner cover layer is harder than the outer cover layer, the inner coverlayer having a Shore D hardness of at least 60 (or at least about 80Shore C) as measured on the surface thereof, and a softer outer coverlayer comprising thermoplastic polyurethane, polyurea orpolyurethane/polyurea formed over the inner cover layer, the outer coverlayer having a Shore C hardness of less than 98, preferably a Shore Chardness of 95 or less, more preferably 90 or less, as measured on thesurface thereof, the golf ball cover having improved scuff resistance.

In another aspect, the present invention provides a golf ball comprisinga core, a hard inner cover layer formed over the core, and a softerouter cover layer formed over the inner cover layer. The inner coverlayer has a Shore D hardness of at least 60 (or at least about 80 ShoreC) as measured on the curved surface thereof and is formed of acomposition including at least one material selected from the group ofconsisting of ionomers (10-100% neutralization), polyamides,polyurethanes, polyureas, polyester elastomers, polyester amides,metallocene catalyzed polyolefins, and blends thereof. The outer coverlayer has a Shore C hardness of less than 98, preferably a Shore Chardness of 95 or less, more preferably 90 or less, as measured on thecurved surface thereof. It is formed from a composition comprising atleast thermoplastic polyurethane, polyurea or polyurethane/polyureamaterial.

The golf ball precursor products utilized with the invention can be of astandard or enlarged size. The core may have multiple layers, such as adual core having a spherical center or inner core and a core layersurrounding the inner core. Additional core layers may also be present.The cover layer is preferably a multi-layer cover comprising at least aninner cover layer and an outer cover, although any number of coverlayers, such as 2, 3, 4, 5 or more is possible.

The core or the dual core of the golf ball precursor product can be,formed of a solid, a liquid, or any other substance that will result inan inner ball (core and inner cover layer), having the desired COR,compression and hardness. The multi-layered cover preferably comprisestwo layers: a first or inner layer or ply and a second or outer layer orply. The inner layer can be ionomer, ionomer blends, non-ionomer,non-ionomer blends, or blends of ionomer and non-ionomer. The outerlayer is preferably softer than the inner layer and can be thermoplasticpolyurethane, polyurea, polyurethane/polyurea blends, or a blend of apolyurethane/polyurea and ionomer or non-ionomer.

In a further embodiment, the inner layer is comprised of a hard, highacid (i.e. greater than 16 weight percent acid) ionomer resin or highacid ionomer blend. Preferably, the inner layer is comprised of a blendof two or more high acid (i.e. at least 16 weight percent acid) ionomerresins neutralized to various extents by different metal cations. Theinner cover layer may or may not include a metal stearate (e.g., zincstearate) or other metal fatty acid salt. The purpose of the metalstearate or other metal fatty acid salt is to lower the cost ofproduction without affecting the overall performance of the finishedgolf ball. In an additional embodiment, the inner layer is comprised ofa hard, low acid (i.e. 16 weight percent acid or less) ionomer blend.Preferably, the inner layer is comprised of a blend of two or more lowacid (i.e. 16 weight percent acid or less) ionomer resins neutralized tovarious extents by different metal cations. The inner cover layer may ormay not include a metal stearate (e.g., zinc stearate) or other metalfatty acid salt.

It has been found that a hard inner layer provides for a substantialincrease in resilience (i.e., enhanced distance) over known multi-layercovered balls. The softer outer layer provides for desirable “feel” andhigh spin rate while maintaining respectable resiliency. The soft outerlayer allows the cover to deform more during impact and increases thearea of contact between the clubface and the cover, thereby impartingmore spin on the ball. As a result, the soft cover provides the ballwith a balata-like feel and playability characteristics with improveddistance and durability. Consequently, the overall combination of theinner and outer cover layers results in a golf ball having enhancedresilience (improved travel distance) and durability (i.e. cutresistance, etc.) characteristics while maintaining and in manyinstances, improving, the playability properties of the ball.

The combination of a hard inner cover layer or layers with a soft,relatively low modulus thermoplastic polyurethane, polyurea orpolyurethane/polyurea outer cover layer provides for excellent overallcoefficient of restitution (for example, excellent resilience) becauseof the improved resiliency produced by the inner cover layer. While someimprovement in resiliency is also produced by the outer cover layer, theouter cover layer generally provides for a more desirable feel and highspin, particularly at lower swing speeds with highly lofted clubs suchas half wedge shots.

Preferably, the inner cover layer is harder than the outer cover layerand generally has a thickness in the range of 0.010 to 0.150 inches,preferably 0.010 to 0.100 inches, more preferably 0.020 to 0.060 inchesfor a 1.68 inch ball and 0.030 to 0.100 inches for a 1.72 inch (or more)ball. The core and inner cover layer together form an inner ball havinga coefficient of restitution at 125 feet per second of 0.750 or more andmore preferably 0.790 or more, and a diameter in the range of 1.48 to1.67 inches for a 1.68 inch ball and 1.50 to 1.71 inches for a 1.72 inch(or more) ball. The inner cover layer has a Shore D hardness of 60 ormore (or at least about 80 Shore C). It is particularly advantageous ifthe golf balls of the invention have an inner layer with a Shore Dhardness of 65 or more (or at least about 100 Shore C). If the innerlayer is too thin, it is very difficult to accurately measure the ShoreD, and sometimes the Shore C, of the inner layer as the layer maypuncture. Additionally, if the core is harder, this will sometimesinfluence the reading. If the Shore C or Shore D is measured on a plaqueof material, different values will result. The above-describedcharacteristics of the inner cover layer provide an inner ball having aPGA compression of 100 or less. It is found that when the inner ball hasa PGA compression of 90 or less, excellent playability results.

The inner layer compositions of the embodiments described herein mayinclude the high acid ionomers such as those developed by E.I. DuPont deNemours & Company under the trademark Surlyn® and by Exxon Corporationunder the trademarks Escor® or Iotek®, or blends thereof.

The high acid ionomers which may be suitable for use in formulating theinner layer compositions of various embodiments of the invention areionic copolymers which are the metal, (such as sodium, zinc, magnesium,etc.), salts of the reaction product of an olefin having from about 2 to8 carbon atoms and an unsaturated monocarboxylic acid having from about3 to 8 carbon atoms. Preferably, the ionomeric resins are copolymers ofethylene and either acrylic or methacrylic acid. In some circumstances,an additional comonomer such as an acrylate ester (for example, iso- orn-butylacrylate, etc.) can also be included to produce a softerterpolymer. The carboxylic acid groups of the copolymer are partiallyneutralized (for example, approximately 10-100%, preferably 30-70%) bythe metal ions. Each of the high acid ionomer resins which may beincluded in the inner layer cover compositions of the invention containsgreater than about 16% by weight of a carboxylic acid, preferably fromabout 17% to about 25% by weight of a carboxylic acid, more preferablyfrom about 18.5% to about 21.5% by weight of a carboxylic acid.

The high acid ionomeric resins available from Exxon under thedesignation Escor® or Iotek®, are somewhat similar to the high acidionomeric resins available under the Surlyn® trademark. However, sincethe Escor®/Iotek® ionomeric resins are sodium or zinc salts ofpoly(ethylene-acrylic acid) and the Surlyn® resins are zinc, sodium,magnesium, etc. salts of poly(ethylene-methacrylic acid), distinctdifferences in properties exist.

Examples of the high acid methacrylic acid based ionomers found suitablefor use in accordance with this invention include Surlyn® 8220 and 8240(both formerly known as forms of Surlyn® AD-8422), Surlyn® 9220 (zinccation), Surlyn® SEP-503-1 (zinc cation), and Surlyn® SEP-503-2(magnesium cation). According to DuPont, all of these ionomers containfrom about 18.5 to about 21.5% by weight methacrylic acid.

Examples of the high acid acrylic acid based ionomers suitable for usein the present invention also include the Escor® or Iotek® high acidethylene acrylic acid ionomers produced by Exxon such as Ex 1001, 1002,959, 960, 989, 990, 1003, 1004, 993, 994. In this regard, Escor® orIotek® 959 is a sodium ion neutralized ethylene-acrylic neutralizedethylene-acrylic acid copolymer. According to Exxon, Ioteks® 959 and 960contain from about 19.0 to about 21.0% by weight acrylic acid withapproximately 30 to about 70 percent of the acid groups neutralized withsodium and zinc ions, respectively.

Furthermore, as a result of the development by the assignee of thisapplication of a number of high acid ionomers neutralized to variousextents by several different types of metal cations, such as bymanganese, lithium, potassium, calcium and nickel cations, several highacid ionomers and/or high acid ionomer blends besides sodium, zinc andmagnesium high acid ionomers or ionomer blends are now available forgolf ball cover production. It has been found that these additionalcation neutralized high acid ionomer blends produce inner cover layercompositions exhibiting enhanced hardness and resilience due tosynergies that occur during processing. Consequently, the metal cationneutralized high acid ionomer resins recently produced can be blended toproduce substantially higher C.O.R.'s than those produced by the lowacid ionomer inner cover compositions presently commercially available.

More particularly, several metal cation neutralized high acid ionomerresins have been produced by the assignee of this invention byneutralizing, to various extents, high acid copolymers of analpha-olefin and an alpha, beta-unsaturated carboxylic acid with a widevariety of different metal cation salts. This is the subject matter ofU.S. Pat. No. 5,688,869, incorporated herein by reference. It has beenfound that numerous metal cation neutralized high acid ionomer resinscan be obtained by reacting a high acid copolymer (i.e. a copolymercontaining greater than 16% by weight acid, preferably from about 17 toabout 25 weight percent acid, and more preferably about 20 weightpercent acid), with a metal cation salt capable of ionizing orneutralizing the copolymer to the extent desired (for example, fromabout 10% to 90%).

The base copolymer is made up of greater than 16% by weight of an alpha,beta-unsaturated carboxylic acid and an alpha-olefin. Optionally, asoftening comonomer can be included in the copolymer. Generally, thealpha-olefin has from 2 to 10 carbon atoms and is preferably ethylene,and the unsaturated carboxylic acid is a carboxylic acid having fromabout 3 to 8 carbons. Examples of such acids include acrylic acid,methacrylic acid, ethacrylic acid, chloroacrylic acid, crotonic acid,maleic acid, fumaric acid, and itaconic acid, with acrylic acid beingpreferred.

The softening comonomer that can be optionally included in the innercover layer for the golf ball of the invention may be selected from thegroup consisting of vinyl esters of aliphatic carboxylic acids whereinthe acids have 2 to 10 carbon atoms, vinyl ethers wherein the alkylgroups contains 1 to 10 carbon atoms, and alkyl acrylates ormethacrylates wherein the alkyl group contains 1 to 10 carbon atoms.Suitable softening comonomers include vinyl acetate, methyl acrylate,methyl methacrylate, ethyl acrylate, ethyl methacrylate, butyl acrylate,butyl methacrylate, or the like.

Consequently, examples of a number of copolymers suitable for use toproduce the high acid ionomers included in the present inventioninclude, but are not limited to, high acid embodiments of anethylene/acrylic acid copolymer, an ethylene/methacrylic acid copolymer,an ethylene/itaconic acid copolymer, an ethylene/maleic acid copolymer,an ethylene/methacrylic acid/vinyl acetate copolymer, anethylene/acrylic acid/vinyl alcohol copolymer, etc. The base copolymerbroadly contains greater than 16% by weight unsaturated carboxylic acid,from about 39 to about 83% by weight ethylene and from 0 to about 40% byweight of a softening comonomer. Preferably, the copolymer containsabout 20% by weight unsaturated carboxylic acid and about 80% by weightethylene. Most preferably, the copolymer contains about 20% acrylic acidwith the remainder being ethylene.

Along these lines, examples of the preferred high acid base copolymerswhich fulfill the criteria set forth above, are a series ofethylene-acrylic copolymers which are commercially available from TheDow Chemical Company, Midland, Mich., under the Primacor® designation.

The metal cation salts utilized in the invention are those salts thatprovide the metal cations capable of neutralizing, to various extents,the carboxylic acid groups of the high acid copolymer. These includeacetate, oxide or hydroxide salts of lithium, calcium, zinc, sodium,potassium, nickel, magnesium, and manganese.

Examples of such lithium ion sources are lithium hydroxide monohydrate,lithium hydroxide, lithium oxide and lithium acetate. Sources for thecalcium ion include calcium hydroxide, calcium acetate and calciumoxide. Suitable zinc ion sources are zinc acetate dihydrate and zincacetate, a blend of zinc oxide and acetic acid. Examples of sodium ionsources are sodium hydroxide and sodium acetate. Sources for thepotassium ion include potassium hydroxide and potassium acetate.Suitable nickel ion sources are nickel acetate, nickel oxide and nickelhydroxide. Sources of magnesium include magnesium oxide, magnesiumhydroxide, and magnesium acetate. Sources of manganese include manganeseacetate and manganese oxide.

The metal cation neutralized high acid ionomer resins are produced byreacting the high acid base copolymer with various amounts of the metalcation salts above the crystalline melting point of the copolymer, suchas at a temperature from about 200° F. to about 500° F., preferably fromabout 250° F. to about 350° F. under high shear conditions at a pressureof from about 10 psi to 10,000 psi. Other well known blending techniquesmay also be used. The amount of metal cation salt utilized to producethe new metal cation neutralized high acid based ionomer resins is thequantity which provides a sufficient amount of the metal cations toneutralize the desired percentage of the carboxylic acid groups in thehigh acid copolymer. The extent of neutralization is generally fromabout 10% to about 90%.

A number of different types of metal cation neutralized high acidionomers can be obtained from the above-indicated process. These includehigh acid ionomer resins neutralized to various extents with manganese,lithium, potassium, calcium and nickel cations. In addition, when a highacid ethylene/acrylic acid copolymer is utilized as the base copolymercomponent of the invention and this component is subsequentlyneutralized to various extents with the metal cation salts producingacrylic acid based high acid ionomer resins neutralized with cationssuch as sodium, potassium, lithium, zinc, magnesium, manganese, calciumand nickel, several cation neutralized acrylic acid based high acidionomer resins are produced.

When compared to low acid versions of similar cation neutralized ionomerresins, the new metal cation neutralized high acid ionomer resinsexhibit enhanced hardness, modulus and resilience characteristics. Theseare properties that are particularly desirable in a number ofthermoplastic fields, including the field of golf ball manufacturing.

When utilized in the construction of the inner layer of a multi-layeredgolf ball, it has been found that the acrylic acid based high acidionomers extend the range of hardness beyond that previously obtainablewhile maintaining the beneficial properties (i.e. durability, click,feel, etc.) of the softer low acid ionomer covered balls, such as ballsproduced utilizing the low acid ionomers disclosed in U.S. Pat. Nos.4,884,814 and 4,911,451. By using these high acid ionomer resins,harder, stiffer inner cover layers having higher C.O.R.s, and thuslonger distance, can be obtained.

More preferably, it has been found that when two or more of theabove-indicated high acid ionomers, particularly blends of sodium andzinc high acid ionomers, are processed to produce the covers ofmulti-layered golf balls, (for example, the inner cover layer or layersherein) the resulting golf balls will travel further than previouslyknown multi-layered golf balls produced with low acid ionomer resincovers due to the balls' enhanced coefficient of restitution values.

Alternatively, if the inner cover layer comprises a low acid, the lowacid ionomers which may be suitable for use in formulating the innerlayer compositions of the subject invention are ionic copolymers whichare the metal, (sodium, zinc, magnesium, etc.), salts of the reactionproduct of an olefin having from about 2 to 8 carbon atoms and anunsaturated monocarboxylic acid having from about 3 to 8 carbon atoms.Preferably, the ionomeric resins are copolymers of ethylene and eitheracrylic or methacrylic acid. In some circumstances, an additionalcomonomer such as an acrylate ester (for example, iso- orn-butylacrylate, etc.) can also be included to produce a softerterpolymer. The carboxylic acid groups of the copolymer are partiallyneutralized (for example, approximately 10 to 100%, preferably 30 to70%) by the metal ions. Each of the low acid ionomer resins which may beincluded in the inner layer cover compositions of the invention contains16% by weight or less of a carboxylic acid.

The inner layer compositions include the low acid ionomers such as thosedeveloped and sold by E.I. DuPont de Nemours & Company under thetrademark Surlyn® and by Exxon Corporation under the trademarks Escor®or Iotek®, or blends thereof.

The low acid ionomer resins available from Exxon under the designationEscor® and/or Iotek®, are somewhat similar to the low acid ionomericresins available under the Surlyn® trademark. However, since theEscor®/Iotek® ionomeric resins are sodium or zinc salts ofpoly(ethylene-acrylic acid) and the Surlyn® resins are zinc, sodium,magnesium, etc. salts of poly(ethylene-methacrylic acid), distinctdifferences in properties exist.

When utilized in the construction of the inner layer of a multi-layeredgolf ball, it has been found that the low acid ionomer blends extend therange of compression and spin rates beyond that previously obtainable.More preferably, it has been found that when two or more low acidionomers, particularly blends of sodium and zinc ionomers, are processedto produce the covers of multi-layered golf balls, (for example, theinner cover layer herein) the resulting golf balls will travel furtherand at an enhanced spin rate than previously known multi-layered golfballs. Such an improvement is particularly noticeable in enlarged oroversized golf balls.

In one embodiment of the inner cover layer, a blend of high and low acidionomer resins is used. These can be the ionomer resins described above,combined in a weight ratio which preferably is within the range of 10 to90 to 90 to 10 high and low acid ionomer resins.

Another embodiment of the inner cover layer is primarily or fullynon-ionomeric thermoplastic material. Suitable non-ionomeric materialsinclude metallocene catalyzed polyolefins or polyamides,polyamide/ionomer blends, polyphenylene ether/ionomer blends, etc.,which have a Shore D hardness of at least 60 (or at least about 80 ShoreC) and a flex modulus of greater than about 15,000, more preferablyabout 30,000 psi, or other hardness and flex modulus values which arecomparable to the properties of the ionomers described above. Othersuitable materials include but are not limited to thermoplastic orthermosetting polyurethanes, thermoplastic block polyesters, forexample, a polyester elastomer such as that marketed by DuPont under thetrademark Hytrel®, or thermoplastic block polyamides, for example, apolyether amide such as that marketed by Elf Atochem S.A. under thetrademark Pebax®, a blend of two or more non-ionomeric thermoplasticelastomers, or a blend of one or more ionomers and one or morenon-ionomeric thermoplastic elastomers. These materials can be blendedwith the ionomers described above in order to reduce cost relative tothe use of higher quantities of ionomer.

A golf ball inner cover layer according to the present invention formedfrom a polyurethane material typically contains from about 0 to about 60weight percent of filler material, more preferably from about 1 to about30 weight percent, and most preferably from about 1 to about 20 weightpercent.

While the core with the hard inner cover layer formed thereon providesthe multi-layer golf ball with power and distance, the outer cover layeris preferably comparatively softer than the inner cover layer. Thesoftness provides for the feel and playability characteristics typicallyassociated with balata or balata-blend balls. The outer cover layer orply is comprised of a relatively soft, low modulus (about 1,000 psi toabout 100,000 psi, preferably about 5,000 psi to about 70,000)thermoplastic polyurethane, polyurea or polyurethane/polyurea, or ablend of two or more polyurethanes, or a blend of one or more ionomersor one or more non-ionomeric thermoplastic materials with athermoplastic polyurethane. The outer layer is 0.005 to about 0.150 inchin thickness, preferably 0.010 to 0.075 inch in thickness, moredesirably 0.015 to 0.040 inch in thickness, but thick enough to achievedesired playability characteristics while minimizing expense. Thicknessis defined as the average thickness of the non-dimpled areas of theouter cover layer. The outer cover layer preferably has a Shore Chardness of less than 98, preferably 95 or less, and more preferably 90or less, as measured on the surface of the golf ball. If the inner layerand/or core are harder than the outer layer, this will sometimesinfluence the reading. If the Shore C is measured on a plaque ofmaterial, different values may result.

The outer cover layer of the invention is formed over a core to resultin a golf ball having a coefficient of restitution of at least 0.750,more preferably at least 0.780, and most preferably at least 0.790. Thecoefficient of restitution of the ball will depend upon the propertiesof both the core and the cover. The PGA compression of the golf ball is100 or less, and preferably is 90 or less.

In polyurethanes, cross-linking can occur in a number of ways includingbetween the isocyanate groups (—NCO) and the polyol's hydroxylend-groups (—OH), and/or with already formed urethane groups.Additionally, the end-use characteristics of polyurethanes can also becontrolled by different types of reactive chemicals and processingparameters. For example, catalysts are utilized to controlpolymerization rates.

Generally, thermoplastic polyurethanes have some cross-linking, butprimarily by physical means. The cross-link bonds can be reversiblybroken by increasing temperature, as occurs during molding or extrusion.In this regard, thermoplastic polyurethanes can be injection molded, andextruded as sheet and blow film. They can be used up to about 350° F. to500° F. and are available in a wide range of hardnesses.

The thermoplastic polyurethane, polyurea or polyurethane/polyurea whichis selected for use as a golf ball cover preferably has a Shore Chardness of from about 98 or less, more preferably about 95 or less, andmost preferably about 90 or less when measured on the surface of thegolf ball. The thermoplastic polyurethane, polyurea orpolyurethane/polyurea which is to be used for a cover layer preferablyhas a flex modulus from about 1 to about 310 Kpsi, more preferably fromabout 5 to about 100 Kpsi, and most preferably from about 5 to about 70Kpsi. Accordingly, covers comprising these materials exhibit similarproperties.

Non-limiting examples of a polyurethane, polyurea orpolyurethane/polyurea suitable for use in the outer cover layer includethermoplastic polyurethanes available from Bayer under the trade name ofTEXIN and DESMOPAN, polyurethanes available from BASF under the tradename ELLASTOLAN, polyurethanes available from Dow under the trade namePELLETHANE, and polyurethanes from Noveon Incorporated, such as58132-XLK-040; 58132-XCT-040; 58134-XL2-040P; 58134-XL4-040P;58134-XC2-040P; 58134-XC4-040P; 5740x960-XL2; 5740x960-XL4;5740x960-XC2; and 5740x960-XC4.

Typically, there are two classes of thermoplastic polyurethanematerials: aliphatic polyurethanes and aromatic polyurethanes. Thealiphatic materials are produced from a polyol or polyols and aliphaticisocyanates, such as H₁₂MDI or HDI, and the aromatic materials areproduced from a polyol or polyols and aromatic isocyanates, such as MDIor TDI. The thermoplastic polyurethanes may also be produced from ablend of both aliphatic and aromatic materials, such as a blend of HDIand TDI with a polyol or polyols.

Generally, the aliphatic thermoplastic polyurethanes are lightfast,meaning that they do not yellow appreciably upon exposure to ultravioletlight. Conversely, aromatic thermoplastic polyurethanes tend to yellowupon exposure to ultraviolet light. One method of stopping the yellowingof the aromatic materials is to paint the outer surface of the finishedball with a coating containing a pigment, such as titanium dioxide, sothat the ultraviolet light is prevented from reaching the surface of theball. Another method is to add UV absorbers and stabilizers to the clearcoating(s) on the outer cover, as well as to the thermoplasticpolyurethane material itself. By adding UV absorbers and stabilizers tothe thermoplastic polyurethane and the coating(s), aromaticpolyurethanes can be effectively used in the outer cover layer of golfballs. This is advantageous because aromatic polyurethanes typicallyhave better scuff resistance characteristics than aliphaticpolyurethanes, and the aromatic polyurethanes are typically lower costthan aliphatic polyurethanes.

A golf ball outer cover layer according to the present invention formedfrom a polyurethane material typically contains from about 0 to about 20weight percent of filler material, more preferably from about 1 to about10 weight percent, and most preferably from about 1 to about 5 weightpercent.

Moreover, in alternative embodiments, either the inner and/or the outercover layer may also additionally comprise up to 100 wt % of a soft, lowmodulus, non-ionomeric thermoplastic or thermoset material.Non-ionomeric materials are suitable so long as they produce theplayability and durability characteristics desired without adverselyaffecting the enhanced travel distance characteristic produced by thehigh acid ionomer resin composition. These include but are not limitedto styrene-butadiene-styrene block copolymers, including functionalizedstyrene-butadiene-styrene block copolymers,styrene-ethylene-butadiene-styrene (SEBS) block copolymers such asKraton® materials from Shell Chem. Co., and functionalized SEBS blockcopolymers; metallocene catalyzed polyolefins; ionomer/rubber blendssuch as those in U.S. Pat. Nos. 4,986,545; 5,098,105 and 5,187,013;silicones; and Hytrel® polyester elastomers from DuPont and Pebax®polyetheramides from Elf Atochem S.A. A preferred non-ionomeric materialsuitable for the inner and/or outer cover layer includes polyurethane.

Additional materials may also be added to the inner and outer coverlayer of the present invention as long as they do not substantiallyreduce the playability properties of the ball. Such materials includedyes (for example, Ultramarine Blue™ sold by Whittaker, Clark, andDaniels of South Plainsfield, N.J.) (see U.S. Pat. No. 4,679,795);pigments such as titanium dioxide, zinc oxide, barium sulfate and zincsulfate; UV absorbers; antioxidants; antistatic agents; and stabilizers.Moreover, the cover compositions of the present invention may alsocontain softening agents such as those disclosed in U.S. Pat. Nos.5,312,857 and 5,306,760, including plasticizers, metal stearates,processing acids, etc., and reinforcing materials such as glass fibersand inorganic fillers, as long as the desired properties produced by thegolf ball covers of the invention are not impaired.

Examples of suitable isocyanates include, but are not limited to,4,4′-diphenylmethane diisocyanate (“MDI”); 2,4-toluene diisocyanate(“TDI”); m-xylylene diisocyanate (“XDI”); methylene bis-(4-cyclohexylisocyanate) (“HMDI”); hexamethylene diisocyanate (“HDI”);naphthalene-1,5,-diisocyanate (“NDI”); 3,3′-dimethyl-4,4′-biphenyldiisocyanate (“TODI”); 1,4-diisocyanate benzene (“PPDI”);phenylene-1,4-diisocyanate; and 2,2,4- or 2,4,4-trimethyl hexamethylenediisocyanate (“TMDI”). Other less preferred diisocyanates include, butare not limited to, isophorone diisocyanate (“IPDI”); 1,4-cyclohexyldiisocyanate (“CHDI”); diphenylether-4,4′-diisocyanate; p,p′-diphenyldiisocyanate; lysine diisocyanate (“LDI”); 1,3-bis (isocyanato methyl)cyclohexane; polymethylene polyphenyl isocyanate (“PMDI”); andmeta-tetramethylxylylene diisocyanate (“TMXDI”). Preferably, thediisocyanate is MDI. The term “isocyanate” as used herein includes allof these compounds and other isocyanates.

As mentioned generally above, the isocyanate or diisocyanate used mayhave a solids content in the range of about 1 to about 100 weight %,preferably about 5 to about 50 weight %, most preferably about 10 toabout 30 weight %. If it is necessary to adjust the solids content, anysuitable solvent (such as ketone and acetate) that will allowpenetration of the isocyanate into the polyurethane cover materialwithout causing distortion may be used.

More preferably, the isocyanate utilized is Mondur ML™, an aromaticdiisocyanate manufactured by the Bayer Corporation. According to Bayer,Mondur ML™ is an isomer mixture of diphenyl methane diisocyanate (MDI)containing a high percentage of 2,4 isomer. More particularly, MondurML™ reportedly has the following specifications and proportions:

A. PRODUCT SPECIFICATIONS Assay, wt. % 99.5 minimum Acidity as HCI, ppm30 maximum 2′,4′ isomer content, % 50-60 Dimer, wt. % 0.3 maximum

B. TYPICAL PROPERTIES* Appearance Clear to light yellow liquidEquivalent weight 125 NCO Content, % 33.4-33.6 Viscosity @25° C., 10mPa*s Weight per gallon, lb 9.9 @25° C. Specific Gravity @ 25° C. 1.19Freezing point 59°-68° F. (15-20° C.) Flash point (Setaflash) 388° F.(198° C.) Equivalent wt., avg. (as supplied) 125 *These items areprovided as general information only. They are approximate values andare not considered part of the product specification.

The cores of the inventive golf balls typically have a coefficient ofrestitution of about 0.750 or more, more preferably 0.770 or more and aPGA compression of about 90 or less, and more preferably 70 or less.Furthermore, in some applications it may be desirable to provide a corewith a coefficient of restitution of about 0.780 to 0.790 or more. Thecore used in the golf ball of the invention preferably is a solid. Theterm “solid cores” as used herein refers not only to one piece cores butalso to those cores having a separate solid layer beneath the covers andover the central core. The cores have a weight of 25 to 40 grams andpreferably 30 to 40 grams. When the golf ball of the invention has asolid core, this core can be compression molded from a slug of uncuredor lightly cured elastomer composition comprising a high cis contentpolybutadiene and a metal salt of an α, β, ethylenically unsaturatedcarboxylic acid such as zinc mono- or diacrylate or methacrylate. Toachieve higher coefficients of restitution and/or to increase hardnessin the core, the manufacturer may include a small amount of a metaloxide such as zinc oxide. In addition, larger amounts of metal oxidethan are needed to achieve the desired coefficient may be included inorder to increase the core weight so that the finished ball more closelyapproaches the U.S.G.A. upper weight limit of 1.620 ounces. Non-limitingexamples of other materials which may be used in the core compositionincluding compatible rubbers or ionomers, and low molecular weight fattyacids such as stearic acid. Free radical initiator catalysts such asperoxides are admixed with the core composition so that on theapplication of heat and pressure, a curing or cross-linking reactiontakes place.

A thread wound core may comprise a liquid, solid, gel or multi-piececenter. The thread wound core is typically obtained by winding a threadof natural or synthetic rubber, or thermoplastic or thermosettingelastomer such as polyurethane, polyester, polyamide, etc. on a solid,liquid, gel or gas filled center to form a thread rubber layer that isthen covered with one or more mantle or cover layers. Additionally,prior to applying the cover layers, the thread wound core may be furthertreated or coated with an adhesive layer, protective layer, or anysubstance that may improve the integrity of the wound core duringapplication of the cover layers and ultimately in usage as a golf ball.

In a preferred embodiment, the final golf ball precursor producttypically is coated with a durable, abrasion-resistant, relativelynon-yellowing finish coat or coats if necessary. The finish coat orcoats may have some optical brightener added to improve the brightnessof the finished golf ball. In a preferred embodiment, from 0.001 toabout 10% optical brightener may be added to one or more of the finishcoatings. Preferred finish coatings are solvent based urethane coatingsknown in the art.

After molding, the golf balls produced may undergo various furtherprocessing steps such as buffing, painting and marking as disclosed inU.S. Pat. No. 4,911,451.

The resulting golf ball produced from the hard inner layer and therelatively softer, low flexural modulus outer layer which additionallycomprises a functionalized polybutadiene and/or an isocyanate providefor an improved multi-layer golf ball which provides for desirablecoefficient of restitution and durability properties while at the sametime offering the feel and spin characteristics associated with softbalata and balata-like covers of the prior art.

The golf balls formed according to the present invention can be coatedusing a conventional two-component spray coating or can be coated duringthe process, for example, using an in-mold coating process.

The present invention includes a wide variety of strategies andtechniques for improving the scuff resistance of thermoplasticpolyurethane covers.

The present invention is further illustrated by the following examplesin which the parts of the specific ingredients are by weight. It is tobe understood that the present invention is not limited to the examples,and various changes and modifications may be made in the inventionwithout departing from the spirit and scope thereof.

EXAMPLES

Golf balls precursor products comprising thermoplastic polyurethanecovers were made and subjected to a first dipping in a solutioncontaining a functionalized polybutadiene, and a second dipping in aisocyanate solution. The results are shown in Table One below.

The scuff resistance test was conducted in the manner described below.The balls that tested were primed and top coated. A 56° Wedge (256·12)was mounted in a mechanical swing machine. The club swing speed used is70 mph. After each hit, the club face is brushed clean using a nylonbristled brush. A minimum of three samples of each ball were tested.Each ball was hit three times at three different locations so as not tooverlap with other strikes. The details of the club face are critical,and are as follows:

Groove width 0.026 inches Groove depth 0.014 inches;

For each strike, a point value is assigned based on a scale from 0.0 to6.0 with 0.0 representing no visible mark from the strike and 6.0representing shredding of the material, with consideration given to apotential end user's perception of cover damage. After completing allstrikes, determine the average point value. This average point value, orrank, can be correlated to the chart below.

Scuff Test Ranking Rank Average Point Value Excellent 0.0-1.0 Very Good1.1-2.0 Good 2.1-3.0 Fair 3.1-4.0 Borderline 4.1-5.0 Poor (unacceptable)5.1-6.0

The cut test (off center cut) was performed as described below. An offcenter cut test was used as it more closely represents actual play. Theshear component of this blow makes the off-cut center cut test the mostsevere and most useful in determining the cut resistance of a covermaterial.

TABLE ONE 1^(st) 1^(st) 2^(nd) 2^(nd) Example Solution Dip Time HeatingTime Dip Time Heating Time Scuff 1 16% MDI in Acetone 2 min. 4 hrs. @175 F. NA NA 3.5 (Control) 2 1. 20% KRASOL LBH 2000 2 min. Air Dry @Room Temp. 2 min. 4 hrs. @ 175 F. 3.1 2. 16% MDI in Acetone 3 1. 20%KRASOL LBH 2000 2 min. 4 hrs. @ 175 F. 2 min. 4 hrs. @ 175 F. 3.3 2. 16%MDI in Acetone 4 1. 16% MDI in Acetone 2 min. 4 hrs. @ 175 F. 2 min. 4hrs. @ 175 F. 3.3 2. 20% KRASOL LBH 2000

Golf balls precursor products comprising a cover composed of athermoplastic polyurethane a functionalized polybutadiene material weresubjected to a dipping in an isocyanate solution and then gammaradiation. The results are shown in Table Two below.

TABLE TWO Iso Dip and Post-Bake (per Gamma Example Cover MaterialExample 1) Irradiation Scuff 5 Control TPU Yes 0 3.0 6 Control TPU Yes20 KGYS 3.6 7 Control TPU Yes 40 KGYS 3.7 8 TPU with 1.5% Krasol Yes 20KGYS 2.4 HLBH-P3000 9 TPU with 1.5% Krasol Yes 40 KGYS 2.5 HLBH-P3000 10TPU with 1.5% Krasol Yes 20 KGYS 2.4 NN22 11 TPU with 1.5% Krasol Yes 40KGYS 2.5 NN22

From the foregoing it is believed that those skilled in the pertinentart will recognize the meritorious advancement of this invention andwill readily understand that while the present invention has beendescribed in association with a preferred embodiment thereof, and otherembodiments illustrated in the accompanying drawings, numerous changes,modifications and substitutions of equivalents may be made thereinwithout departing from the spirit and scope of this invention which isintended to be unlimited by the foregoing except as may appear in thefollowing appended claims. Therefore, the embodiments of the inventionin which an exclusive property or privilege is claimed are defined inthe following appended claims.

1. A method of forming a golf ball, the method comprising: placing agolf ball precursor product in a solution to create a solution coveredgolf ball precursor product, the golf ball precursor product comprisinga cover comprising a thermoplastic polyurethane material, the solutioncomprising a hydroxyl terminated polybutadiene in an amount of 10% to30% by weight of the solution and a solvent; removing the solutioncovered golf ball precursor product from the solution; heating thesolution covered golf ball precursor product to remove the solvent tocreate a pre-treated solution covered golf ball precursor product;placing the pre-treated solution covered golf ball precursor product inan isocyanate solution to create a isocyanate solution covered golf ballprecursor product; and heating the isocyanate solution covered golf ballprecursor product to create a final golf ball precursor product.
 2. Themethod according to claim 1 wherein the golf ball precursor product isplaced in the solution for one to two minutes.
 3. The method accordingto claim 1 wherein the solution covered golf ball precursor product isheated from two to four hours at a temperature ranging from 125° F. to250° F.
 4. The method according to claim 1 wherein the pre-treatedsolution covered golf ball precursor product is placed in the isocyanatesolution for one to two minutes.
 5. The method according to claim 1wherein the isocyanate solution covered golf ball precursor product isheated from two to four hours at a temperature ranging from 125° F. to250° F.
 6. The method according to claim 1 wherein heating the solutioncovered golf ball precursor product comprises air-drying the solutioncovered golf ball precursor product at approximately 72° F.
 7. Themethod according to claim 1 wherein the solvent is acetone.
 8. Themethod according to claim 1 wherein the isocyanate solution comprisesacetone and MDI.
 9. The method according to claim 1 wherein the solutioncomprises 20 weight percent of the hydroxyl terminated polybutadiene.10. The method according to claim 1 wherein the solvent is selected fromthe group consisting of methyl ethyl ketone and toluene.
 11. The methodaccording to claim 1 wherein the isocyanate solution comprises MDI in anamount of sixteen weight percent of the isocyanate solution and acetone.12. The method according to claim 1 wherein the final golf ballprecursor product is subjected to gamma irradiation.